A major drawback in the utilization of hydrogen-based fuel cells for powering vehicles or microelectronic devices is the lack of an acceptable lightweight, high-capacity, and safe hydrogen storage medium. Four conventional approaches to hydrogen storage are currently in use: (a) liquid hydrogen, (b) compressed gas, (c) cryo-adsorption, and (d) metal hydride or chemical hydride storage systems. A brief description of these existing approaches is given below:    (a) The liquid hydrogen storage approach offers good solutions in terms of technology maturity and economy, for both mobile storage and large-volume storage systems with volumes ranging from 100 liters to 5000 m3. However, the containers for storing the liquefied hydrogen are made of very expensive super-insulating materials.    (b) The compressed gas storage approach is usually applied in underground supply systems, similar to a network of natural gas pipelines. This is an economical and simple approach, but it is unsafe and not portable. Compressed hydrogen gas in a large steel tank could be an explosion hazard.    (c) The cryo-adsorbing storage approach involves moderate weight and volume. In this approach, hydrogen molecules are bound to the sorbent only by physical adsorption forces, and remain in the gaseous state. The adsorbing temperature is in the range of 60 to 100° K. Activated carbon is commonly used as the sorbent due to its large number of small pores serving as hydrogen storage sites. The efficiency of H2 uptake is no more than 7 wt %, which is equivalent to about 20 kg H2 per cubic meter of activated carbon. The disadvantages of this approach are related to the low capacity and the cryogenic temperature required, which makes it necessary to use expensive super-insulated containers.    (d) The metal hydrides can store H2 via a chemical reaction of H+M≈M−H, wherein M is a selected metal element. Two major metal systems, i.e. Fe—Ti and Mg—Ni, have been applied as hydrogen storage media and have been put into use in automobiles driven by a H2/O2 fuel cell. The operating temperature is 40-70° C. for the Ti—Fe system and 250-350° C. for the Mg—Ni system. The hydrogen storage capacity is less than 5 wt % for Ni—Mg and 2 wt % for Fe—Ti, which corresponds to less than 70 kg H2 per m3 of metals. Furthermore, metal hydride systems normally require 20-40 bar pressure to keep the hydrogen in equilibrium. This renders the container for the metal hydride too heavy and expensive, and limits the practical exploitation of these systems for portable electronic and mobility applications.
Another class of hydrogen storage materials is based on the storage technologies in which hydrogen is generated through a chemical reaction such as hydrolysis and hydrogenation-dehydrogenation. Common reactions involve chemical hydrides with water or alcohols. Typically, these reactions are not easily reversible on-board a vehicle. Hence, the spent fuel and/or byproducts must be removed from the vehicle and regenerated off-board. Hydrolysis reactions involve the oxidation reaction of chemical hydrides with water to produce hydrogen. This prior art approach is well summarized in the following patent literature:    1. S. Suda, “Method for Generation of Hydrogen Gas,” U.S. Pat. No. 6,358,488 (Mar. 19, 2002).    2. S. C. Amendola, et al., “System for Hydrogen Generation,” U.S. Pat. No. 6,534,033 (Mar. 18, 2003).    3. S. C. Amendola, et al., “Portable Hydrogen Generator,” U.S. Pat. No. 6,932,847 (Aug. 23, 2005).    4. C. A. Lumsden, et al., “Aqueous Borohydride Compositions,” U.S. Pat. No. 6,866,689 (Mar. 15, 2005).    5. R. M. Mohring, et al., “System for Hydrogen Generation,” U.S. Pat. No. 7,083,657 (Aug. 1, 2006).
As an example, the reaction for sodium borohydride is:NaBH4+2H2O=NaBO2+4H2.  (1)
In real practice, a slurry of an inert stabilizing liquid is used to protect the hydride from contact with moisture. At the moment of actual use, the slurry is mixed with water and the consequent reaction produces high purity hydrogen. In another approach, sodium borohydride is dissolved in water, which is stabilized by a caustic salt such as NaOH or KOH to form a solution. The alkaline state of the solution prevents the dissolved sodium borohydride from decomposing and prematurely releasing hydrogen gas. Hydrogen is generated on demand by bringing this alkaline solution to contact a metal catalyst such as platinum or ruthenium. This prior art approach has the following major drawbacks: (1) the need to use an expensive catalyst such as platinum; (2) a limited solubility of NaBH4 in water at a given temperature and the need to include water in the fuel container, resulting in a reduced amount of NaBH4 that can be accommodated in a container (hence, a low energy density); and (3) the need to use a caustic ingredient that makes the handling of the fuel more difficult (corrosion-resistant container required) and complicates the process for recovering the spent fuel such as NaBO2.
A potentially effective approach to utilizing a chemical hydride such as sodium borohydride as a hydrogen source is to begin with a hydrogen-generating process with a dry, solid chemical hydride separate from a liquid phase such as water or alkaline solution (not allowing sodium borohydride to be dissolved in an aqueous alkaline solution). Upon demand from a hydrogen powered device such as a fuel cell, the complex hydride is brought to contact with water and a metal catalyst. This approach may be represented by the following patents:    6. P. A. Kerrebrock, et al., “Hydrogen Generation by Hydrolysis of Hydrides for Undersea Vehicle Fuel Cell Energy Systems,” U.S. Pat. No. 5,372,617 (Dec. 13, 1994).    7. S. W. Jorgensen, “Method of Generating Hydrogen From Borohydrides and Water,” U.S. Pat. No. 6,866,836 (Mar. 15, 2005).    8. S. H. Kravitz, et al., “Compact Solid Source of Hydrogen Gas,” U.S. Pat. No. 6,746,496 (Jun. 8, 2004).
There are still problems associated with this approach. For instance, adding water to commercially available powders, granules, or pellets of solid sodium borohydride, in the presence of a metal catalyst, such as ruthenium, results in caking and scaling of the borohydride surface due to production of the reactant product sodium metaborate (NaBO2) in the form of a surface layer (i.e., crust or scale). As the scale layer grows progressively thicker, it gets increasingly more difficult for the water to penetrate through the metaborate crust to reach the unreacted NaBH4 fuel below, resulting in a decreased hydrogen production rate. Kravitz, et al., [Ref. 8 cited above] attempted to solve the problem of crust formation (i.e., caking, scaling) of the surface of solid sodium borohydride particles, granules, or pellets from the reaction product sodium metaborate (i.e., borax) during hydrogen production by using micro-disperse particles of solid sodium borohydride. The micro-disperse particles are in the form of microspheres having a diameter of 1-100 microns. The water only has to diffuse through a very thin layer of sodium metaborate to totally react each fuel particle to completion. However, this approach requires the incorporation of nanometer-scaled metal particles as a catalyst in the micro-disperse particles. Nano catalyst particles such as platinum, ruthenium, and cobalt are very expensive. They are also difficult to recover and reuse once they become part of the spent fuel.
The above review indicates that the hydrogen storage technology still has the following major barriers to overcome: (1) low H2 storage capacity, (2) difficulty in storing and releasing H2 (normally requiring a high temperature to release and a high pressure to store), (3) high costs, (4) potential explosion danger, (5) need to utilize expensive catalysts, and (6) utilization of caustic solution. A need exists for a new high-capacity medium that can safely store and release hydrogen at near ambient temperature and pressure conditions. More specifically, what is further needed is a material and system for generating hydrogen gas that utilizes a metal hydride or a chemical hydride, such as solid sodium borohydride, in a highly efficient manner that prevents caking and scaling from reducing the hydrogen production yield and without using an expensive metal catalyst.
It may be noted that Kaufman, et al [“Hydrogen Generation by Hydrolysis of Sodium Tetrahydroborate: Effects of Acids and Transition Metals and their Salts,” Journal of Chem. Soc., Dalton Trans. (1985) 307-313] studied the effect of an acid solution or metal salt solution on the hydrolysis of sodium borohydride in a solution form. However, their study did not utilize (nor did they suggest explicitly or implicitly the utilization of) a dry, solid acid powder in an admixture with solid sodium borohydride, which is found by us to be a very convenient form of reactant delivery. By contrast, our invented approach entails delivering a dry, solid acid that is either pre-mixed with solid sodium borohydride (or other chemical hydride or metal hydride) at a desired proportion, or mixed with solid sodium borohydride (or other chemical hydride or metal hydride) at a proportion that varies with time according the changing hydrogen consumption need of a fuel cell. The mixture is delivered, on demand, to contact with a reactant liquid (e.g., water or alcohol). Furthermore, the study of Kaufman, et al. was limited to sodium borohydride, not including any other type of chemical hydride or any simple metal hydride. They also failed to use environmentally benign acids or metal salts to accelerate the hydrogen generation reaction. For instance, they used perchloric acid which is undesirable. This is an important consideration since the hydrogen generation herein discussed is intended for use by billions of people on a daily basis in their automobiles, motorcycles, microelectronic devices, etc. The only liquid reactant used by Kaufman, et al. was water. By contrast, we have also surprisingly observed that metal hydride reactions with an alcohol (methanol, ethanol, etc.) can be accelerated by using a small amount of very inexpensive acid or metal salt.
Hence, an object of the present invention is to provide a method that features a high hydrogen storage capacity and an ability to safely and reliably store and feed hydrogen fuel to a power-generating device such as a combustion engine or fuel cell.
Another object of the present invention is to provide a method that is capable of storing hydrogen in a metal hydride or chemical hydride and releasing the hydrogen fuel in a controlled manner without involving an excessively high heating temperature and without using an expensive metal catalyst.
Still another object of the present invention is to provide a hydrogen storage and supply material that is particularly suitable for feeding hydrogen fuel to fuel cells for use in apparatus such as portable electronic devices, automobiles, specialty vehicles, and unmanned aerial vehicles (UAV) where device weight is a major concern.